FIG. 1 illustrates a typical prior art railroad grade crossing 100 with a single railroad track 102. A first gate 104A and 104B is closed when a train approaches on track 102 thereby restricting the flow of traffic from the corresponding side of track 102. A second gate 106A and 106B is closed on the opposite side of track 102 from gates 104A and 104B to restrict the flow of traffic from the opposite side.
In FIG. 2, a similar prior art railroad grade crossing 200 is shown but with two tracks 202 and 204 shown as the grade crossing 200. Similar to shown above for the single track configuration 100, a first gate 206A and 206B is closed when a train approaches on track 202 or 204 thereby restricting the flow of traffic from that side of track 102. A second gate 208A and 208B is closed on the opposite side of tracks 202 and 204 from gates 206A and 206B to restrict the flow of traffic from the opposite side.
In these prior art systems, the gates close when an approaching train is detected. In order to detect obstacles located between closed gates in the proximity of the tracks, some prior art systems rely on a transmitter/receiving system that is responsive to reflections of the transmitted signals by the obstacles themselves and do not utilize a reflector or detect the presence of a signal from the reflector. See U.S. Pat. No. 6,340,139 and U.S. Pat. No. 5,625,340.
Other prior art systems rely on reflectors that reflect frequency-modulated radar which utilize the frequency and amplitude differences between the transmitted and reflected signal to determine the presence of an object in the surveillance zone. These prior art systems detect differences in signal amplitude and the signal phase. The latter results from a phase shift determined by the signal transit time as defined by a transit time component at the reflector. However, in this known implementation, the system includes a receiver, circulator, transit time element, a directional separating filter, and an amplifier, each of which incrementally adds to the complexity and cost of the system. See U.S. Pat. No. 5,775,045.
Several systems have been developed which utilize microwave detection systems. However, prior art systems currently encounter problems such as false detection of obstacles, inaccurate detection of obstacles, failure to detect obstacles, detection of echoes, inadequate surveillance, and high cost associated with the initial installation and with ongoing operations.
Corresponding reference characters and designations generally indicate corresponding parts throughout the drawings.